1. Field of the Invention
The present invention relates to a ball end mill of the type having an indexable cutter insert with convexly curved cutting edges releasably attached to an outer periphery of a forward end portion of an end mill body.
2. Related Art
FIGS. 1 to 3 show one related art ball end mill of the aforedescribed type as disclosed in Japanese Utility Model Application No. 62-107230 of the same applicant.
The ball end mill includes a generally cylindrical end mill body 10 having a generally hemispherical forward end portion and a rearward end portion which is adapted to be fixedly secured to a machine spindle so that the end mill body 10 can be rotated about an axis therethrough. A pair of diametrically opposed chip pockets 12 and 14 are formed in the forward end portion of the end mill body 10, and a recess 16, 18 is formed in that wall surface of each chip pocket 12, 14 which faces in the direction of rotation of the body 10. Indexable first and second cutter inserts 20 and 22 are releasably mounted in the recesses 16 and 18, respectively.
Out of these two inserts, the first insert 20 is so disposed as to be slightly offset forwardly of the end mill body 10 from the second insert 22. As shown in FIGS. 4 to 6, the insert 20 comprises a plate of a generally elliptical shape defined by a front face 24, a rear face 26 disposed parallel to the front face 24, and two pairs of first and second side faces 28 and 30 disposed alternately and lying between the front and rear faces 24 and 26, each side face being sloping inwardly of the plate in a direction away from the front face 24. The two first side faces 28 as well as the two second side faces 30 are opposed to each other so as to be symmetrical with respect to an axis of the plate or insert. Each first side face 28 is convexly curved so as to assume a quadrantlike shape as viewed in a direction perpendicular to the front face 24 while each second side face 30 is planar and very short as compare with the first side face 28. Each first side face and each second side face intersect each other so as to define an obtuse angle therebetween. The insert has a pair of convexly curved main cutting edges 32 each defined by the intersection of a respective one of the first side faces 28 with the front face 24, and a pair of short cutting edges 34 each defined by the intersection of a respective one of the second side faces 30 with the front face 24. Each main cutting edge 32 has a leading end where it intersects one of the short cutting edges 34 and also has a trailing end where it intersects the other short cutting edge 34, and an angle defined by the short cutting edge 28 and a line tangent to the main cutting edge 26 at its leading end is obtuse. As will be apparent from FIG. 3 one of the short cutting edges 34 passes through the axis of rotation of the body 10.
The front face 24 serves as a rake surface for the cutting edges, and a marginal surface portion 36 thereof disposed adjacent to each short cutting edge 34 and the adjacent leading and trailing end portions of the main cutting edges 32 intersecting the short cutting edge 34 is convexly arcuate in such a manner as to slope toward the rear face 26 toward the short cutting edge 34, whereby the leading and trailing end portions of each main cutting edge 32 are convexly curved as viewed in a side elevation. The insert plate has a central aperture 38 formed therethrough and extending from the front face 24 to the rear face 26. The aperture 38 has a greater diameter portion 38a disposed adjacent to the front face 24, a reduced diameter portion 38b disposed adjacent to the rear face 26 and a tapered portion 38c connecting the larger diameter portion 38a and the reduced diameter portion 38b together.
The second insert 22 also includes a pair of convexly curved main cutting edges 40, but its construction will not be described in detail. Each insert 20, 22 is received in a respective one of the recesses 16 and 18 and releasably secured thereto by a clamp screw 42 in such a manner that one of the main cutting edges 32, 40 is indexed in a working position so as to extend slightly beyond the forward end portion of the body 10. As best shown in FIG. 7, the clamp screw 42 is inserted through the central aperture 38 of the insert and threaded into the body 10 with its head portion 42a held against the tapered portion 38c of the aperture 38 and with its threaded portion 42b threadedly engaged with a threaded bore 44 formed in a bottom face 16a of the recess 16. Further, the insert 20 is retained in position with the rear face 26 mated with a bottom face 16a of the recess 16 and with a portion of the first side face 28 defining the other main cutting edge 32 held in abutment with a portion of an arcuately shaped side wall 16b of the recess 16.
In the related art ball end mill as described above, since the leading end portion of the main cutting edge 32 is convexly curved as viewed axially of the end mill body 10, the cutting load increases gradually as the cutting operation proceeds, and therefore a great cutting load will not be exerted on the cutting edge 32 at a time. Besides, inasmuch as the angle defined by the short cutting edge 34 and the line tangent to the main cutting edge 32 at its leading end is obtuse, the indexed cutting edge 32 has an increased strength even at its leading end. Further, the ball end mill also has advantages that it possesses a better chip-disposal ability in comparison with conventional ball end mills.
In the related art ball end mill as described above, the portion of the side wall 16b of the recess 16 undergoes a cutting load F exerted on the insert 20 in an axial direction as shown in FIG. 1. If the insert should be of a triangular or quadrilateral shape, a bearing surface on which the load F is exerted perpendicularly could be formed on the side wall of the recess. However, inasmuch as the insert 20 is of an elliptical shape, such bearing surface cannot be provided. Accordingly, when the ball end mill is fed in the axial direction at a high speed, the insert 20 is pressed axially rearwardly of the body along the side wall 16b, and that portion of the cutting edge indexed in an end cutting position is caused to shift a distance b axially rearwardly of the body while that portion of the cutting edge indexed in a peripheral cutting position is caused to shift a distance a radially outwardly of the body. As a result, the cutting accuracy is unduly lowered particularly in the peripheral cutting position.